A translator or interpreter was an individual who converted a phrase in one language into its equivalent in another. The advantage of having an individual as a translator was that the emotions intended by a speaker could be expressed, not merely the literal translation of a phrase. (TNG: "Loud As A Whisper")
In notes that Costume Designer Robert Fletcher wrote about the various aliens in the filmStar Trek: The Motion Picture, he included a description of a Federation "diplomatic service" consisting of translators. The brief stated, "In San Francisco scene, may be Earthlings or aliens. Interpreters, translators. Have three-unit machine through which he can speak or hear any foreign tongue. Can set machine for any language. Right side of jacket has miniaturized components which measure nuances in language translators. Portable computer on left arm. Helmet allows to hear and make sounds during translating." (The Making of Star Trek: The Motion Picture, p. 133)
In an interview with Robert Fletcher – published in the February 1980 edition of Fantastic Films, as part one of a two-part interview with him, called "The Star Trek Costumes" – Fletcher stated the then-forthcoming filmStar Trek: The Motion Picture would include a "diplomatic service" of "interpreters, translators for all diplomatic events." He went on to say, "The service is made up of both Humans and aliens. Each interpreter wears a small machine comprised of three units through which they can not only hear but speak any language known to the Federation."